1. Field of the Invention
This invention relates to an improved signal processing circuit for a movement detecting encoder device which tracks the movement of a body by means of sensors which track the movement of a scale attached to the body.
Such movement detecting encoders are well known in the art, as for example the magnetic rotary encoder disclosed in U.S. Pat. No. 4,774,464. A similar example of a magnetic rotary encoder is shown in FIG. 5a. A magnetic drum 50 is provided with a scale, which is an array of magnetic elements 51, that produces a changing magnetic field as magnetic drum 51 rotates. The changing magnetic field is detected by MR sensors 1a, 1b. Signal processing circuits for such encoders are also well known in the art, for example, as disclosed in U.S. Pat. No. 4,359,685. FIGS. 5 shows MR sensors 1a, 1b in detail in the context of a similar typical signal processing circuit made up of phase A and phase B. MR sensor 1a comprises magnetoresistive elements Ra1-Ra4 configured in a bridge circuit, and is provided with source voltage V.sub.cc. Further, MR sensor 1a is connected to phase A of the signal processing circuit by means of sensor output nodes P1 and P2 Sensor output P1 is connected to the inverting terminal of comparator 2 a by means of resistor R1a, and sensor output P2 is connected to the non-inverting terminal by means of resistor R2a.
The construction of MR sensor 1b and its connection to phase B of the signal processing circuit mirrors that of MR sensor 1a. Typically, the output of the phase B output has a phase difference of 90.degree. (.pi./2) with the phase A output, and both signals are typically further processed in signal processing means such as a detection signed generator 3.
As shown in FIG. 6b, e1 and e2 represent typical sensor output waveforms at nodes P1 and P2, respectively. These waveforms are 180.degree. out of phase with each other and intersect at action reference voltage V.sub.cc /2 Comparator 2a is provided with source voltage V.sub.cc =V.sub.cc in this example, so that the output of the comparator 2a alternates between V.sub.cc and 0 v (ground) and similarly has action reference voltage V.sub.cc /2. The output A of the comparator is fed back to the noninverting terminal of comparator 2a through a feedback resistor R3a causing a voltage displacement in the input waveform to the non-inverting terminal of comparator 2a, from e2 to e3. The voltage amplitude R2a, R3a, the sensor output action reference voltage V.sub.2 and the comparator 2a output voltage V.sub.A according to the following equation:
(1) Amplitude displacement=(V.sub.A -V.sub.2).times.[R2a/(R2a+R3a)] When the comparator output is high, V.sub.A is approximately equal to V.sub.cc. As mentioned earlier, the action reference voltage V.sub.2 of sensor output waveform e2 is V.sub.cc /2, so when the comparator output is high the amplitude displacement is: ##EQU1## When the comparator output is low, V.sub.A is approximately 0, and the amplitude displacement is: ##EQU2## Thus, the magnitude of the amplitude displacement when the source voltage of the comparator is equal to the source voltage of the sensors is the same whether the comparator output is high or low; only the sign of the amplitude displacement changes.
However, when the source voltage V.sub.cc2 of the comparator is different from the source voltage V.sub.cc of sensor 1a, the magnitude of the amplitude displacement changes as well. This problem arises in prior art due to the positive feedback to the non-inverting terminal of comparator 2a through resistor R3a, which is provided to lessen the effect of noise that is typically present in a sensor output waveform as shown in FIG. 8.
For example, suppose V.sub.cc2 equals V.sub.cc /3 as in FIG. 7. Waveform e3, the input to the non-inverting terminal of comparator 2a, is formed as before, according to equation (1). However, since V.sub.cc2 is not equal to V.sub.cc, the magnitude of factor V.sub.A -V.sub.cc will change when V.sub.A toggles between its high and low values. When the output of the comparator is high, amplitude displacement is given by: EQU V.sub.fh2 =(V.sub.cc /3-V.sub.cc /2).times.[R2a/(R2a+R3a)]
When the comparator output is low amplitude displacement is given by: EQU V.sub.fL2 =(0-V.sub.cc /2).times.[R2a/(R2a+R3a)]
Since V.sub.fH2 =(V.sub.cc /3-V.sub.cc /2).noteq.(0-V.sub.cc /2)=V.sub.fL2 the amplitude displacement changes as the output of the comparator changes.
Differing amplitude displacement is a problem because it causes a distortion of the duty cycle of comparator 2a output. The duty cycle is not distorted when the source voltage V.sub.cc2 of comparator is equal to the source voltage of MR sensor 1a because the amplitude displacement is the same.
An undistorted duty cycle means that the time interval while the comparator is in the high output level or low output level accurately represents the time interval between appropriate intersection points of the sensor output waveforms. For example, referring to FIG. 6, two "output waveform intersection points" i.e. the intersection points of the sensor output waveforms e1 and e2, are labelled x1 and x2. Between x1 and x2, along the x-axis, output waveform e1, the input to the inverting terminal of comparator 2a, is greater than the output waveform e2, the input to the non-inverting terminal of comparator 2a. Therefore the comparator output V.sub.A should be low for this time interval. However, because of positive feedback, waveform e3 rather than e2 is the input to the non-inverting terminal of comparator 2a, so that the comparator output follows the intersection points of waveforms e3 and e1 ("triggering intersection points"), Y1 and Y2, rather than corresponding output intersection points x1 and x2 of waveforms e2 and e1.
Because the amplitude displacement of waveform e3 is equal in magnitude but opposite in sign when the comparator output toggles, the triggering intersection points y1 and y2 are both offset from corresponding output intersection points x1 and x2, respectively, by the same magnitude and in the same direction. Consequently the time interval during which the comparator output is low accurately represents the time interval between the output intersection points x1 and x2, and the duty cycle is not distorted.
However in FIG. 7 where V.sub.cc2 is less than V.sub.cc there is a distortion in the duty cycle. The triggering intersection points y3 and y4 are offset from the corresponding output intersection points x1 and x2, respectively, in different directions because of differing amplitude displacement (V.sub.fL2 .noteq.V.sub.fh2). Thus the time interval between y3 and y4 is greater than the time interval between corresponding output intersection points x1 and x2, respectively. Consequently the time interval of the low comparator output does not accurately represent the time interval between output intersection points x1 and x2, and there is a distortion of the duty cycle.
In summary, a difference between the source voltage V.sub.cc2 of the comparator 2a and the source voltage V.sub.cc of sensor 1a results in a distortion of the duty cycle, or in other words, an inaccurate representation of the sensor output waveforms.